1) Field of the Invention
The present invention is generally related to the underwater laying of pipelines and more particularly to the near vertical laying of pipeline offshore in deep water.
2) General Background
At the present time the offshore pipeline industry utilizes the concept of near vertical pipe laying (called J-Lay) as a system for deep water pipe laying. It has been heretofore proposed to conduct J-Lay pipe laying from a vessel having a tower pivotally mounted to the vessel so that it can be stowed horizontally and raised to a near vertical position during a pipe laying operation. A supply of pipe sections is stowed on the vessel""s deck. Individual pipe sections are successively positioned within the tower by means of a loader termed a strongback. The strongback holds a pipe section to be introduced into the upper end of a pipeline while the lower end of such pipe section is welded to the upper end of the pipeline. The upper end of the pipeline extends through a fixed tower-mounted clamping mechanism located at the lower position of the tower. The tower includes a downward extension that houses pipeline guides and/or tensioners. After such welding is accomplished the pipeline and its newly-added pipe section is lowered through the clamping mechanism and the downward extension of the tower and the strongback is returned to the deck to pick-up a new pipe section. These steps are repeated as the vessel moves forward to thereby effect laying of the pipeline on the bottom of the sea. An example of this type of pipe laying system is shown in U.S. Pat. No. 5,421,675.
The aforedescribed underwater pipe laying systems may be helpful in laying pipe sections of uniform diameter. However, where large or bulky devices must be inserted into the pipeline, these systems do not permit expedient insertions of such items in the pipeline. Examples of such large or bulky devices are pipe valves, tee assemblies with their supporting structures and mud-mats, and pipeline termination assemblies. Pipe valves may have a diameter of 1.23 meters, a flange of 1.3 meters and a height of 1.62 meters from the center of the pipe lengths making up the pipeline, and tee assembly could be over 6 meters in width and 4.5 meters in height. Existing pipe lay systems in the tower do not allow convenient handling of such a bulky device, since it is not practical to design a fixed pipe clamping mechanism which will open up sufficiently wide as to permit downward passage of such devices. Also, where the pipeline extends through a plurality of tensioners it is necessary to successively open such tensioners to permit passage of such bulky devices thereby temporarily loosening tension applied to the pipeline.
The undersea pipe laying method and apparatus of the present invention permits bulky devices to be inserted into the pipeline without requiring a temporary pipeline support and the auxiliary equipment necessitated by the use of such temporary support. The additional time and effort required by the use of such temporary support and auxiliary equipment is eliminated by the present invention thereby effecting considerable cost savings.
More particularly, a preferred embodiment of the method and apparatus of the present invention is utilized with a pipe-laying vessel having a tower and a pivotable strongback which loads pipe into the tower for addition to the upper end of a pipeline which extends into the sea. The upper end of such pipeline is supported by a hang-off clamp or clamp arranged at the floor of the tower below a welding station. A downward extension of the tower houses a pipeline guide and/or a pipeline tensioner. The hang-off clamp may be of the mechanical form locking, friction holding or track tensioner type. Such hang-off clamp is opened to permit downward movement of the pipeline after a new pipe section has been attached by welding to the upper end of such pipeline. The tower is provided with means for supporting and lowering the new pipe section and guide means for aligning the new pipe section with the upper end of the pipeline for attachment to the upper end of the pipeline at the welding station. When it becomes necessary to insert a bulky device, such as a pipe valve or the like into the pipeline, a pipe section containing such device is welded to the upper end of the pipeline while the pipeline is supported by the tower-mounted hang-off clamp. Next, the hang-off clamp and guide means move the pipe section and attached pipeline away from their original position in the tower aligned with the downward extension of the tower to a second position located forwardly and clear of the downward extension. The new pipe section and attached pipeline are then lowered by a hoisting device until the bulky device is disposed below the downward extension. Thereafter, the hoisting device returns the hang-off clamp to its original position on the floor of the tower and another pipe section can then be welded to the upper end of the now-submerged bulky device-containing pipe section.
A second preferred embodiment of the method and apparatus of the present invention differs from the first embodiment in that after a pipe section containing the bulky device has been welded to the upper end of the pipeline a hoisting device moves such pipe section out of the hang-off clamp from the original position to its second position. The hoisting device then lowers the pipe section to the predetermined depth clear of the downward extension of the tower and thereafter returns the upper end of the pipe section to the hang-off clamp so that another pipe section can be added to the upper end of the now-submerged bulky device-containing pipe section.
The method and apparatus of the present invention also permits a pipe section to be added to a pipeline more rapidly than the heretofore utilized pipe laying systems, such as shown in U.S. Pat. No. 5,421,675 since it eliminates the necessity that the strongback support a pipe section while the latter is welded to the pipeline.
These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment which, taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention.